LONDON – Britain raised its terror threat alert to the second-highest level Friday, one of several recent moves the country has made to increase vigilance against international terrorists after a Christmas Day bombing attempt on a Europe-U.S. flight.

The threat level was raised from “substantial” – where it had stood since July to indicate a strong possibility of a terrorist attack – to “severe,” meaning such an attack is considered highly likely.

In making the announcement, Home Secretary Alan Johnson said the raised security level means that Britain is heightening its vigilance. But he stressed that there was no intelligence suggesting an attack is imminent.

“The highest security alert is `critical,’ and that means an attack is imminent, and we are not at that level,” he said on British television.

Johnson declined to say what intelligence the change was based on, or whether the move was related to the failed Christmas bombing attempt, when U.S. authorities say a young Nigerian named Umar Farouk Abdulmutallab tried to detonate a bomb hidden in his underwear during a flight from Amsterdam to Detroit. Abdulmutallab, who allegedly had links to extremists based in Yemen, had studied as a university student in London.

“It shouldn’t be thought to be linked to Detroit, or anywhere else for that matter,” Johnson said. “We never say what the intelligence is.”

He said the decision to raise the threat level was made by the U.K.’s Joint Terrorism Analysis Center. He said the center kept the security threat level under constant review and made its judgments based on a range of factors, including the “intent and capabilities of international terrorist groups in the U.K. and overseas.”

Friday’s changes came days after Britain suspended direct flights to Yemen’s capital in response to the growing threat from al-Qaida-affiliated militants based in that country. Prime Minister Gordon Brown said his government also was creating a new terrorist no-fly list, and targeting specific airline passengers for tougher security checks.

The measures followed a discussion between Brown and President Barack Obama on Tuesday. They match similar moves made by U.S. authorities last week to enhance security at airports and on planes, as intelligence officials warned that al-Qaida’s branch in Yemen was continuing to plot attacks on the United States.

The stepped-up security in the U.S. included more air marshals on flights to and within the U.S. and additional screening at airports around the world.

Brown said Britain and other nations face a sharply growing threat from al-Qaida-affiliated terrorists based in Yemen and an area of north Africa that includes nations such as Somalia, Nigeria, Sudan and Ethiopia.

Officials and analysts say Britain’s new alert level could be related to the emergence of a steady stream of threat information since the thwarted Christmas Day attack.

In Washington, a senior U.S. official said late Friday that the British move would have followed a specific threat, but the official would not discuss details.

However, the official said the United States did not believe the heightened alert was related to upcoming conferences the British government is hosting on Yemen and Afghanistan next week in London.

U.S. Secretary of State Hillary Rodham Clinton is to attend those meetings on Wednesday and Thursday and those plans remain unchanged, the official said. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Meanwhile, a Capitol Hill official told The Associated Press the intelligence community has detected increased terrorist “chatter” so far in 2010- that is, conversations and messages that suggest a possible elevated level of activity or planning.

But several said they know of no new specific threat that led to the British action. Instead, they noted that the British had lowered their threat level several months ago and were likely raising it to reflect the U.S. government’s threat level.

The U.S. officials all spoke on condition of anonymity because they were not authorized to discuss foreign intelligence publicly.

Britain’s five-tiered alert system – which starts at “low” and passes through “moderate,” “substantial,” and “severe” before hitting “critical” – is similar to the U.S. system of color-coded terrorism advisories.

The British government downgraded the alert level to “substantial” in July without explaining the decision. The level last stood at “critical” in June 2007, after authorities foiled car bomb attacks on a London nightclub and a Scottish airport.

In the United States, the alert level for the aviation sector is currently at “orange,” indicating a high risk of terrorist attacks. It has not been changed since 2006, after terrorist plans to blow up jetliners en route to the U.S. from Britain were discovered. The alert level for the rest of the country is at “yellow,” indicating a significant risk.

Britain’s decision to raise its terror threat alert came as India put airline passengers through extra security screenings and sky marshals were placed on flights. India put its airports on high alert amid reports that al-Qaida-linked militants planned to hijack a plane.